High Temperature Structural Materials Research Articles

Design of Refractory High Entropy Alloys TiZrHfNbX (X=Cr/Ta) system, using thermodynamic calculations

Refractory high entropy alloys (RHEAs) are materials with novel concept that do not use a specific element as a base material and mixture of multiple elements in near equal amounts. This concept is expected to be adopted for use in aerospace heat-resistant materials that can be used at higher temperatures than currently available. We focused on the TiZrHfNbCr system as a new high-temperature structural material. We carried out thermodynamic calculation using the CALPHAD method and proposed compositions of TiZrHf(NbCr)2 and TiZrHf(NbCr)7 in addition to the equimolar composition (TiZrHfNbCr). These alloys are fabricated by arc melting, and microstructures in TiZrHf(NbCr)7 alloys, predicted by CALPHAD were observed in SEM images. Although some agglomeration of Nb is recognized by observation, microstructures of TiZrHf(NbCr)2 were also good agreement with the prediction by CALPHAD. From the results of TiZrHfNbCr, TiZrHf(NbCr)2 and TiZrHf(NbCr)7, complex oxides were formed and small weight gains were observed compared to other alloys (e.g., ZrTi and NbTa). The effect of formation of complex oxide to the oxidation behavior of these alloys will be discussed in the presentation.

Investigation on thermoelectric materials of Ni-Al-M system Heusler alloy

In this study, we investigated the electronic structure and DOS of Ni-Ti-Al Heusler alloys, which have use in research and development as high-temperature structural materials, for high thermoelectromotive force by using first-principles calculations. Based on these analyses, we aim to create and analyze materials with non-stoichiometric compositions and nano- and composite structures in material experiments. The goal is to realize practical high-performance Ni-Ti-Al Heusler alloy thermoelectric materials by searching for materials that exhibit synergistic effects from electronic structure and DOS to nano- and micro-composites on a multiscale and by elucidating the mechanisms.

Excellent strength-ductility synergy of NiAl-based composites achieved by a 3-dimensional network structure

NiAl intermetallic compounds have attracted the attention of researchers in the field of high-temperature structural materials, but their poor room-temperature ductility and insufficient high-temperature strength crying out for solutions have hindered their practical application. To solve these problems, a novel 3-dimensional network structure-reinforced NiAl-based composite was designed by hot presing sintering sinter with pre-alloyed powders. The 3-dimensional network structure was composed of discrete nearly spherical HfO 2 and rod-shaped HfRe 2 . The mechanical properties of the composites were closely correlated with the distribution of the 3-dimensional network structure in the matrix, which was controlled by the sintering temperature and holding time. A microstructure with a desirable 3-dimensional network structure distribution and the best comprehensive mechanical properties was produced at 1375 °C/60 min. The microstructural evolution along with the strengthening and toughening mechanisms were discussed in detail. The increase in the high-temperature strength was mainly attributed to the pinning effect of the 3-dimensional network structure on dislocations and the restraining effect on the sliding and rotation between the matrix grains, while the increase in room-temperature ductility was mainly due to the grain refinement and hindering of crack propagation by the 3-dimensional network structure. In this paper, the development of a novel low-density NiAl-based composite with a 3-dimensional network structure improved both the strength and ductility, providing design ideas for the application of NiAl-based materials.

Insights into high thermal stability of laser additively manufactured Al2O3/GdAlO3/ZrO2 eutectic ceramics under high temperatures

Laser additive manufacturing techniques have been considered to be the most promising methods to fabricate melt-grown Al 2 O 3 -based eutectic ceramics which are potential candidates for high temperature structural applications. In this paper, Al 2 O 3 /GdAlO 3 /ZrO 2 ternary eutectic ceramic was additively manufactured based on one-step melt-growth by laser directed energy deposition, and its thermal stability under high temperatures was studied. The results indicate that the as-solidified eutectic composite consists of ultrafine phases of α-Al 2 O 3 , GdAlO 3 and t-ZrO 2 , and presents an irregular eutectic morphology with amounts of terminations and branches. The 3D-printed eutectic ceramic exhibits a superior thermal stability that no mass variation and no phase transition occur even after annealed at 1500 °C for 200 h. In addition, the submicron-scaled eutectic microstructure and the polished surface topography can maintain stability up to 1400 °C which is higher than 80% of its melting point (>0.8 T m ). With further increasing the temperature, obvious microstructure coarsening, and surface holes formation occur, leading to the severely deterioration of the mechanical property. The hardness of the as-deposited eutectic ceramic rapidly decreases from 15.84 ± 0.61 GPa to 13.01 ± 0.40 GPa after 200 h annealed at 1500 °C. The results indicate that the additively manufactured Al 2 O 3 -based eutectic ceramics have great potential to be long-time high-temperature structural materials applied at elevated temperatures. • Highly dense Al 2 O 3 /GdAlO 3 /ZrO 2 eutectic ceramic is 3D-printed based on melt growth. • Thermal stability of the additively manufactured eutectic ceramic is studied under high temperatures. • Critical transition temperature from stability to instability is determined, and the instability mechanism is revealed.

Microstructure and Oxidation Behavior of Nb-Si-Based Alloys for Ultrahigh Temperature Applications: A Comprehensive Review

Nb-Si-based superalloys are considered as the most promising high-temperature structural material to replace the Ni-based superalloys. Unfortunately, the poor oxidation resistance is still a major obstacle to the application of Nb-Si-based alloys. Alloying is a promising method to overcome this problem. In this work, the effects of Hf, Cr, Zr, B, and V on the oxidation resistance of Nb-Si-based superalloys were discussed. Furthermore, the microstructure, phase composition, and oxidation characteristics of Nb-Si series alloys were analyzed. The oxidation reaction and failure mechanism of Nb-Si-based alloys were summarized. The significance of this work is to provide some references for further research on high-temperature niobium alloys.

A ductile Nb40Ti25Al15V10Ta5Hf3W2 refractory high entropy alloy with high specific strength for high-temperature applications

Refractory high entropy alloys (RHEAs) with good high-temperature softening resistance have been revealed as promising candidates for high-temperature structural materials. In this work, a low-density ductile RHEA, Nb40Ti25Al15V10Ta5Hf3W2, has been developed. The RHEA with a BCC matrix and B2 nanoprecipitates exhibits excellent specific yield strength. The compressive specific yield strength (σ0.2/ρ) at 1073 K is as high as 83.2 MPa g−1 cm3. The different deformation behaviors during compression at 1073 K and 1273 K are also identified. The dislocation-dominated deformation provides the initial strain hardening capability, and then microcracks and dynamic recovery accelerate the transition from strain hardening to softening at 1073 K. While the diffusion-controlled dislocation annihilation and continuous dynamic recrystallization (DRX) are the dominant reasons for persistent strain softening at 1273 K. Our work not only reports a promising RHEA with excellent high-temperature properties, but also promotes the development of RHEAs for high-temperature applications.

Microstructure and mechanical properties of multi-phase reinforced Hf-Mo-Nb-Ti-Zr refractory high-entropy alloys

In this paper, Hf-Mo-Nb-Ti-Zr alloy system with single BCC solid solution phase was selected as the matrix alloy. In order to design a new high temperature structural material with excellent comprehensive properties, the multiphase reinforced refractory high entropy alloys was prepared by adding metallic elements of Al or Cr and nonmetallic elements of B, C or Si into Hf-Mo-Nb-Ti-Zr alloys at the same time. The results show that the type and content of newly formed phases are closely related to the mixing enthalpy between alloying elements and the constituent elements of the matrix alloy. This provides a valuable reference for the prediction of phase formation when adding alloying elements to improve the properties of high entropy alloys.

High-temperature mechanical properties and deformation behavior of carbides reinforced TiNbTaZrHf composite

Refractory high entropy alloys (RHEAs) have broad prospects in the field of high-temperature structural materials because of their outstanding mechanical properties at high temperatures. In this work, a novel TiNbTaZrHf based composite was fabricated by powder metallurgy in-situ method. By introducing carbides, the TiNbTaZrHf based composite exhibits an ultra-high yield strength of 2620 MPa at room temperature. Meanwhile, the composite still maintains a high strength of 508 MPa at 1000 °C. The significant strength improvement can be attributed to the load-bearing effect caused by the dispersed carbides with high volume fraction, while the intergranular fracture of carbides is responsible for the plastic instability. When the temperature rises to 1200 °C, the yield strength decreases significantly. The weakening of load-bearing effect and the activation of grain boundary sliding are the main reason for the strength reduction, while interfacial debonding between the matrix and carbides becomes the main failure behavior.

The influence of Grinding Duration to the Cleaning of Tan Rai Aluminum Hydroxide for Preparation of α-Al2O3

Corundum (α-alumina) is widely used and studied as high temperature structural material, electronic packaging, corrosion resistance ceramics and translucent ceramics. The production of high purity alumina play an important role. This paper presents the results of studying the effect of grinding time on the cleaning process of Tan Rai aluminum hydroxide with acetic acid, the aluminum hydroxide dissolution efficiency by HCl solution. At the same time, the results of research on the properties of aluminum oxide made from aluminum hydroxide are presented after refining in a suitable conditions.

Preparation of high purity α-Al2O3 from industrial aluminum hydroxide

Aluminum oxide (alumina; Al2O3) has advantages such as its thermal, chemical, and physical properties when compared with several ceramics materials. It is widely used in ceramics such as firebricks, abrasives and integrated circuit (IC) packages; catalysts; catalyst supports; ion exchange and other fields. Alumina has a complex structure and many crystalline polymorphic phases such as α-Al2O3, βAl2O3, γ-Al2O3, δ-Al2O3, θ-Al2O3, η-Al2O3, κ-Al2O3, χAl2O3 and ρ-Al2O3. Among these phases, α-Al2O3 is widely used and studied as high temperature structural material, electronic packaging, corrosion resistance ceramics and translucent ceramics. This study focus on the preparation of high purity α-Al2O3 from industrial aluminum hydroxide. The results show that after removing Na2O and SiO2 with distilled water, the sample was dissolved in NaOH solution to form NaAlO2 solution. Precipitating of Al(OH)3 from NaAlO2 solution with one of the following substances: HCl (4N), H2SO4 (4N), CH3COOH (4N) or CO2 (45% Vol). The precipitated Al(OH)3 was filtered and washed with distilled water and then dried. Finally, thermally decompose Al(OH)3 at 11000C in 2 hours to form pure α-Al2O3.

Rare Earth Elements Enhanced the Oxidation Resistance of Mo-Si-Based Alloys for High Temperature Application: A Review

Traditional refractory materials such as nickel-based superalloys have been gradually unable to meet the performance requirements of advanced materials. The Mo-Si-based alloy, as a new type of high temperature structural material, has entered the vision of researchers due to its charming high temperature performance characteristics. However, its easy oxidation and even “pesting oxidation” at medium temperatures limit its further applications. In order to solve this problem, researchers have conducted large numbers of experiments and made breakthrough achievements. Based on these research results, the effects of rare earth elements like La, Hf, Ce and Y on the microstructure and oxidation behavior of Mo-Si-based alloys were systematically reviewed in the current work. Meanwhile, this paper also provided an analysis about the strengthening mechanism of rare earth elements on the oxidation behavior for Mo-Si-based alloys after discussing the oxidation process. It is shown that adding rare earth elements, on the one hand, can optimize the microstructure of the alloy, thus promoting the rapid formation of protective SiO2 scale. On the other hand, it can act as a diffusion barrier by producing stable rare earth oxides or additional protective films, which significantly enhances the oxidation resistance of the alloy. Furthermore, the research focus about the oxidation protection of Mo-Si-based alloys in the future was prospected to expand the application field.

Effect of Al content on the microstructure and mechanical properties of γ-TiAl alloy fabricated by twin-wire plasma arc additive manufacturing system

TiAl alloys are considered as promising high temperature structural materials. However, the inherent brittleness makes it difficult to be processed. In the present research, TiAl alloys with different Al content are fabricated successfully using an innovative twin-wire plasma arc additive manufacturing system. The effect of Al level on the phase composition, microstructure characteristics, microhardness and tensile properties of as-deposited TiAl alloy is investigated in detail. The results show that the α2 phase content exhibits increase tendency with the decrease of Al level, and the α2 phase content is higher in the top region than that in the middle region. The tetragonal ratio c/a of γ phase tends to reduce with the decrease of Al content. The microstructure characteristics also present obvious difference for as-deposited TiAl alloys. The lamellar spacing gradually decreases with the decrease of Al concentration. With decreasing Al level, the microhardness of alloy tends to increase. In addition, the as-deposited Ti-48 Al (at.%) alloy exhibits much better tensile properties than other TiAl alloys due to optimum microstructure characteristics. The findings provide a valuable reference for additively manufactured TiAl alloy with proper composition ratio.

Structures and properties of the (NbMoTaW)100−xCx high-entropy composites

Refractory high-entropy alloys exhibit potential applications as high-temperature structural materials. Combining the advantages of high-entropy alloys (HEAs) and high-entropy ceramics (HECs), novel high-entropy composites (NbMoTaW)100−xCx with high compressive strength were synthesized by vacuum arc melting. The effect of C addition on the microstructures and mechanical properties was investigated. Appropriate C addition was effective in enhancing the compressive strength and ductility. The NbMoTaWC alloy showed 2.5 times that of the NbMoTaW HEA in compressive strength. The NbMoTaWC alloy comprised a combination microstructure of body-centered cubic (BCC) structural HEA and face-centered cubic (FCC) structural HEC. By etching the NbMoTaWC alloy, the FCC phase was demonstrated to be HEC. The (NbMoTaW)100−xCx alloys, composed of HEA and HEC, were demonstrated to be high-entropy composites. New functionality and applicability may be discovered in this kind of high-entropy composites with non-metallic alloying elements, such as C, B, N, and Si.

An Investigation of the Miscibility Gap Controlling Phase Formation in Refractory Metal High Entropy Superalloys via the Ti-Nb-Zr Constituent System

Refractory metal high entropy superalloys (RSAs) have been heralded as potential new high temperature structural materials. They have nanoscale cuboidal bcc+B2 microstructures that are thought to form on quenching through a spinodal decomposition process driven by the Ta-Zr or Nb-Zr miscibility gaps, followed by ordering of one of the bcc phases. However, it is difficult to isolate the role of different elemental interactions within compositionally complex RSAs. Therefore, in this work the microstructures produced by the Nb-Zr miscibility gap within the compositionally simpler Ti-Nb-Zr constituent system were investigated. A systematic series of alloys with compositions of Ti5NbxZr95−x (x = 25–85 at.%) was studied following quenching from solution heat treatment and long duration thermal exposures at 1000, 900 and 700 °C for 1000 h. During exposures at 900 °C and above the alloys resided in a single bcc phase field. At 700 °C, alloys with 40–75 at.% Nb resided within a three phase bcc + bcc + hcp phase field and a large misfit, 4.7–5%, was present between the two bcc phases. Evidence of nanoscale cuboidal microstructures was not observed, even in slow cooled samples. Whilst it was not possible to conclusively determine whether a spinodal decomposition occurs within this ternary system, these insights suggest that Nb-Zr interactions may not play a significant role in the formation of the nanoscale cuboidal RSA microstructures during cooling.

Multicomponent Ni-rich high-entropy alloy toughened with irregular-shaped precipitates and serrated grain boundaries

A novel precipitation-strengthened multicomponent Ni-rich Ni46.23Co23Cr10Fe5Al8.5Ti4W2Mo1C0.15B0.1Zr0.02 (at.%) high-entropy alloy (HEA) was designed with strain-hardening after yielding rather than strain-softening even at the temperature of 1000°C. Due to the rapid onset of intergranular cracking, the HEA with spherical precipitates and straight grain boundaries was brittle, exhibiting an inferior tensile strength of ~220 MPa, and an insufficient uniform elongation of only ~1.9%. The serrated-grain-boundary architecture effectively overcomes this intergranular cracking issue. The resultant HEA with irregular-shaped precipitates and serrated grain boundaries showed a brittle-to-ductile transition, giving in a superior strength up to as high as ~260 MPa while maintaining a uniform elongation of ~6.5%. Such an improvement of the strength and ductility has been attributed to the enhanced resistance of the serrated grain boundaries to the intergranular crack nucleation and propagation. Our current results provide a new method for the innovative design of high-temperature structural materials with superior mechanical performances.

Prediction of double transition metal (Cr1−xZrx)2AlC MAX phases as thermal barrier coatings: Insight from density functional theory

AbstractHerein, a high purity (Cr1−xZrx)2AlC solid solutions (with x = 0–1) have been investigated intensively using the first principles density functional theory calculations. The optimized structural parameters were founded to be in good agreement with the experimental results, and increased steadily with rising Zr in (Cr1−xZrx)2AlC solid solutions. The elastic constants Cij and enthalpy formation have been calculated and firmly confirmed that these solid solutions were mechanically and thermodynamically stable. Moreover, the mechanical properties show that the compounds possess brittle behavior with covalent bonding signified that they are relatively hard compared to other MAX phases. Further, the electronic band structures were studied and found a metallic behavior for all the investigated concentrations. Consequently, all the compositions of solid solutions were identified as promising materials for thermal barrier coating and high temperature structural materials due to the high value of melting temperatures and low minimum thermal conductivity.

Enhancing high-temperature strength and ductility of γ-TiAl matrix composites with controllable dual alloy structure

Gamma-TiAl matrix composites can function as promising high-temperature structural materials. In this study, to improve the poor ductility and insufficient strength of composites at high temperatures, they were successfully prepared via powder metallurgy by adding the Ti–6Al–4V alloy. The experimental results indicate that the soft phases of Ti–6Al–4V were surrounded by the hard phase of Ti–48Al–2Cr–2Nb, and the special structure exhibited a unique strain hardening mechanism, which resulted in the high strength and ductility of the composites at high temperature. The 8 wt% Ti–6Al–4V composites were fractured with an elongation approximately 90.65% and tensile strength of 427.75 MPa at 800 °C and 5 × 10−5 s−1; the elongation and tensile strength were both higher than those of single unreinforced Ti–48Al–2Cr–2Nb alloy. Moreover, with an increase in the deformation strain, the deformation behavior started from the Ti–6Al–4V area, which had good ductility. The dislocation movement was hindered by the Ti–48Al–2Cr–2Nb area, and dislocation accumulation and grain deformation occurred, leading to strength increase. The stress concentration and high-density dislocations were stored in the soft phases, which delayed the initiation and propagation of pores and cracks. Moreover, besides grain sliding, evident dislocation activities occurred; therefore, the high ductility of the composites was obtained.

Synthesis and characterizations of (Mg, Co, Ni, Cu, Zn)O high-entropy oxides

High-temperature structural ceramic materials require stability in terms of thermal and mechanical properties. High entropy oxides (HEOs) are among the emerging novel family of advanced ceramic materials with peculiar functional properties. However, their thermal stabilities and mechanical properties are not well investigated. In this work, HEO systems were synthesized from binary oxides of MgO, CoO, NiO, CuO, and ZnO using solid-state reaction method at high temperature, after obtaining the individual oxides through co-precipitation methods. The phase purity of as-synthesized and sintered samples was characterized using X-ray powder diffraction, while the microstructural investigation was performed using Scanning electron microscopy. Mechanical property of the sintered samples at different sintering times and temperatures was investigated and the sample sintered at a sintering temperature of 1200 °C for 15 h sintering time showed a maximum Vickers hardness of about 16 GPa. This result is comparable with some of the hard ceramic materials, and therefore the materials could be a potential candidate for structural applications.

Laser powder-bed fusion of Mo(Si,Al)2 – Based composite for elevated temperature applications

Mo(Si,Al)2-based composites are promising candidates to be used as heating elements, high temperature coatings and structural materials. In this work, powders mixture of 90 wt% MoSi2 and 10 wt% AlSi10Mg was subjected to a laser powder-bed fusion (LPBF) to produce Mo(Si,Al)2-based composites. The effect of process parameters on morphological features, phase composition, densification, hardness and oxidation resistance were examined in detail, and the optimal process window was defined. Thermodynamic calculations were performed to supplement the experimental data. The primary phase in the solidified material is the C40 hexagonal Mo(Si1−x,Alx)2 and the solubility of aluminum is governed by the applied volumetric energy density of the laser. The microstructure of the top surface of the samples is composed of fine and coarse cellular Mo(Si1−x,Alx)2, enveloped with Al and Si in inter-cellular regions. The materials exhibit hardness in 800–840 HV1 range depending on process conditions. The LPBF-prepared samples displayed a stable thin oxide layer formation after 120 h of oxidation. The glassy layer of oxides, which predominantly grow from inter-cellular regions, serves as a protective layer for further oxidation. The oxidation rate is two-orders of magnitude slower compared to the reference MoSi2 sample.

Factor which governs the feature of texture developed during additive manufacturing; clarified from the study on hexagonal C40-NbSi2

C40-NbSi2 with a hexagonal unit cell is focused as a high-temperature structural material. We first attempted the fabrication of the bulk C40-NbSi2 products via selective laser melting (SLM) in additive manufacturing (AM) process. Strong crystallographic texture control wherein <0001> was parallel to the building direction, i.e. development of the so-called basal fiber texture, was achieved in this study. The texture developed in products does not largely vary by changing the scanning strategy, unlike the textures of C11b-MoSi2 with a tetragonal unit cell and a β-Ti alloy with a cubic unit cell. A comparison of these results led us to the conclusion that crystal symmetry, i.e., the multiplicity of the preferential crystal growth direction, is one of the primary factors that governs the features of the textures developed in AM-built materials.